Periodic or random vibrations or shocks can excite the resonant frequencies in various structures, such as disk drive covers, disk drive bases, automobile oil pans, valve covers, etc., which can be problematic due to the resultant formation of undesirable stresses, displacements, fatigue, and even sound radiation or high levels of sound transmission. In addition, these various components (disk drive covers, automobile valve covers, etc.) may also be used as part of an enclosure to prevent acoustical noise from transmitting through the enclosure and are designed to reduce the level of noise passing through. Such undesirable vibrations, shocks or noise sources are typically induced by external or internal forces or noise generators and can be experienced by a wide variety of articles and under a variety of conditions. For example, resonant vibrations can cause significant levels of acoustical noise in a disk drive assembly. This noise can be easily transmittable through a typical monolithic material cover or base casting of the disk drive allowing excessive noise to pass through the material which is undesirable to the operator of the disk drive. The resonant vibrations in the cover or base may also lead to excessive vertical or horizontal displacement of the key mechanical attachment points in the disk drive leading to poor overall disk drive performance and even potential reliability problems. Control of the resonant vibrations and shock in a disk drive are key to optimum performance in the read/write process and quiet operation plus high disk drive reliability.
The preferred known method to reduce resonant vibrations, shock effects and noise transmission or generation is by using viscoelastic damping materials in a design. The viscoelastic damping materials will dissipate the vibrational energy generated by the resonant vibrations thus reducing the negative effects of the excitation source. The viscoelastic materials when used in a design can also reduce the transmitted or generated noise in an article. The viscoelastic materials can be used as an add-on item to the article or more optimally as an inner layer of a laminate structure used to make the article.
One of the largest uses of viscoelastic laminates is in the automobile industry for oil pans, valve covers, and other viscoelastic laminate formed parts or panels. The laminates in these applications offer significant reductions in the acoustical noise transmission and generation escaping from the engine and also reduce the acoustical noise that can enter the passenger compartment in addition to reducing the resonant frequency amplitudes in the articles. These laminates typically have an attachment area by which the laminate part is attached to a base, housing or other structure. The method of attachment of the laminates could be by screws, bolts, nails, rivets, clamps, or other mechanical attachment devices.
One potential problematic area in using the viscoelastic laminates is in the attachment of the laminates. The viscoelastic damping material will stress relax following attachment of the laminate(s) to the structure or base using the screws, bolts, nails, rivets, clamps or other mechanical attachment devices. The attachment devices are used to securely hold the laminates in a specific alignment and under a specific stress or pressure or torque or fastening force. The torque, pressure, stress, or fastening force will tend to relax somewhat normally even in non-laminate structures due to stress relaxation in the fastener material, substrate material, or fastener attachment point to the structure. Thus, in a laminated article, stress relaxation occurs in the fastening system, the laminate, and also to a smaller degree the higher modulus layers of the laminate. The dominate area of stress relaxation is typically the viscoelastic material part of the laminate. Furthermore, variations in temperature above the application temperature of the laminate using the attachments devices can allow the attachment system to stress relax in a shorter period of time.
The stress, torque, pressure, or fastening force in the attachment device prevents the attachment device from loosening during use of the structure the laminate is attached to. If the attachment device is allowed to fall below a critical attachment force, the laminate could become loose allowing the laminate to shift from the desired location. The loose laminate could interfere with other items near it and/or induce misalignment in items attached to the laminate. Furthermore, the fastener devices could loosen to the point where they would no longer support the laminate in a proper alignment leading to a catastrophic failure of the unit to which the laminate is attached.
Methods that have been used to prevent failure of the attachment of the laminate due to the stress relaxation in the viscoelastic layer are discussed below. Certain methods can add cost, processing time, design complexity, etc., or combinations of each which may not be desirable. Operations or designs to reduce the viscoelastic layers stress relaxation after application of the fastener device include those which are disclosed in PCX-9 POLYCORE COMPOSITES.RTM. Physical Properties Sheet, Pre Finish Metals Inc., Polycore Composites.RTM., Elk Grove Village, Ill., such as:
1) The use of automatic Bolt Torque equipment should allow for the entire laminate construction. This fastener attachment method provides for an increased attachment force (torque or pressure) (as compared to a non-laminate material) via the attachment device to the laminate in the attachment area such that after the damping material layer stress relaxes the minimum force required for the application is maintained. The increased torque required initially when attaching the cover to achieve the minimum torque in the screw following stress relaxation can exceed the strength of the screw head-shaft interface, the screw head features (Phillips, Torx, slotted, etc.) that the driver uses to engage the screw and through which the force is applied. In addition, the screw hole tapping or screw features can be stripped in the base or combinations thereof. PA1 2) Another method involves use of a thin input viscoelastic layer in the construction of the laminate to lessen the amount the viscoelastic layer can stress relax. (PCX-9 discloses a layer which is only 0.0254 mm (0.001") thick.) This approach is undesirable as the optimum design of the laminate viscoelastic thickness to reduce resonant vibrations (and reduce acoustical noise generated or transmission) may not be the optimum for the viscoelastic layer thickness in regard to force retention after viscoelastic relaxation. PA1 3) Another method involves retorquing or applying a secondary (or more) re-application of attachment force once the viscoelastic layer has stress relaxed to achieve the desired attachment force. This method is disadvantageous in that it adds cost to the attachment process and is not acceptable in most applications, especially high volume applications where added work in process or secondary operations can significantly increase manufacturing costs. PA1 4) Another method involves the application of heat to the laminate during the attachment device application process. This method to reduce stress relaxation is disadvantageous in that the use of heat during the laminate article attachment is often not practical for a manufacturing process as it will add cost, application complexity, safety concerns if the temperature required is high, and difficulty in monitoring the process. In addition, components, fluids or electronics near the laminate may not allow for the use of heat in the application of the laminate. PA1 5) Another method involves compression of the viscoelastic layer around the area to be torqued during stamping of the laminate. This method of compression may not provide adequate torque retention in all applications. PA1 a first substrate layer and a second substrate layer; PA1 optionally one or more additional substrate layers positioned between said first and second substrate layers; PA1 optionally one or more bonding material layers bonded between a substrate layer and a vibration damping layer, wherein the storage modulus of each bonding material layer is higher than the storage modulus of the viscoelastic material contained in a vibration damping layer to which it is bonded; PA1 wherein the storage modulus of each substrate layer is greater than that of the viscoelastic material in any vibration damping material layer with which it is in contact; PA1 wherein the laminate article has at least one through hole extending completely therethrough, wherein an area of the article which defines at least one through hole is welded via weld(s) such that the first substrate layer is welded to the second substrate layer. PA1 (a) preparing a laminate comprising at least one layer of vibration damping material, the vibration damping material comprising a viscoelastic material, wherein the vibration damping material is positioned between a first substrate layer and a second substrate layer, and optionally one or more additional substrate layers positioned between said first and second substrate layers wherein each substrate layer has a higher storage modulus than the viscoelastic material in any vibration damping material layer with which it is in contact, optionally one or more bonding material layers bonded between a substrate layer and a vibration damping layer, wherein the storage modulus of each bonding material layer is higher than the storage modulus of the viscoelastic material contained in a vibration damping layer to which it is bonded; and PA1 (b) providing at least one through hole in the laminate article; PA1 (c) welding an area of the laminate defining at least one through hole, such that the first substrate layer is welded to the second substrate layer via weld(s), wherein force is optionally applied to a laminate during welding such that the layers of the laminate are in a desired position. PA1 (a) preparing a laminate comprising at least one layer of vibration damping material, the vibration damping material comprising a viscoelastic material, wherein the vibration damping material is positioned between a first substrate layer and a second substrate layer, and optionally one or more additional substrate layers positioned between said first and second substrate layers wherein each substrate layer has a higher storage modulus than the viscoelastic material in any vibration damping material layer with which it is in contact, optionally one or more bonding material layers bonded between a substrate layer and a vibration damping layer, wherein the storage modulus of each bonding material layer is higher than the storage modulus of the viscoelastic material contained in a vibration damping layer to which it is bonded; and PA1 (b) stamping a laminate article out of the laminate wherein the laminate article has at least one through hole therein; PA1 (c) welding an area of the laminate defining at least one through hole, such that the first substrate layer is welded to the second substrate layer via weld(s), wherein force is applied to the laminate during welding such that the substrate layers are in a parallel position. (This force may be applied by a clamp, for example). PA1 a first substrate layer and a second substrate layer; PA1 at least one layer of vibration damping material comprising a viscoelastic material positioned between said first and second substrate layers; PA1 optionally one or more additional substrate layers positioned between said first and second substrate layers; PA1 optionally one or more bonding material layers bonded between a substrate layer and a vibration damping layer, wherein the storage modulus of each bonding material layer is higher than the storage modulus of the viscoelastic material contained in a vibration damping layer to which it is bonded; PA1 wherein the storage modulus of each substrate layer is greater than that of the viscoelastic material in any vibration damping material layer with which it is in contact; PA1 wherein at least a portion of the laminate article periphery is welded via weld(s) such that the first substrate layer is welded to the second substrate layer. PA1 (a) preparing a laminate article comprising at least one layer of vibration damping material, the vibration damping material comprising a viscoelastic material, wherein the vibration damping material is positioned between a first substrate layer and a second substrate layer, and optionally one or more additional substrate layers positioned between said first and second substrate layers wherein each substrate layer has a higher storage modulus than the viscoelastic material in any vibration damping material layer with which it is in contact, optionally one or more bonding material layers bonded between a substrate layer and a vibration damping layer, wherein the storage modulus of each bonding material layer is higher than the storage modulus of the viscoelastic material contained in a vibration damping layer to which it is bonded; and PA1 (b) welding at least a portion of the laminate article periphery such that the first substrate is welded to the second substrate via weld(s), wherein force is optionally applied to the laminate during welding such that the layers of the laminate are in a desired position. PA1 (a) preparing a laminate comprising at least one layer of vibration damping material, the vibration damping material comprising a viscoelastic material, wherein the vibration damping material is positioned between a first substrate layer and a second substrate layer, and optionally one or more additional substrate layers positioned between said first and second substrate layers wherein each substrate layer has a higher storage modulus than the viscoelastic material in any vibration damping material layer with which it is in contact, optionally one or more bonding material layers bonded between a substrate layer and a vibration damping layer, wherein the storage modulus of each bonding material layer is higher than the storage modulus of the viscoelastic material contained in a vibration damping layer to which it is bonded; and PA1 (b) stamping a laminate article out of the laminate; PA1 (c) welding at least a portion of the laminate article periphery such that the first substrate is welded to the second substrate via weld(s), wherein force is applied to the laminate during welding such that the substrate layers are in a parallel position. (This force may be applied by a clamp, for example).
McCutcheon, et al. U.S. Pat. No. 5,691,037 describes vibration damped laminate articles having improved force (torque and/or pressure and/or stress) retention, a method of making one article type and novel tools used to make the one article type. The first laminate article comprises at least one layer of damping material between at least two substrate layers. At least one deformation area is present in the laminate article wherein the substrate(s) are plastically deformed such that they are closer than non-deformed areas of the substrate and wherein the damping material has less mass than in a non-deformed area of the article; the deformation areas providing the areas of good force retention, for an attachment device attached thereto. The second laminate article, which is not deformed, contains an additive of sufficient modulus, diameter and loading, in a vibration damping layer to provide improved force retention.